The present invention relates to a method and apparatus for determining the state of a battery. More specifically, the present invention relates to a prediction and correction method for determining the state of charge of a battery.
In today""s automotive market, there exist a variety of propulsion or drive technologies used to power vehicles. The technologies include internal combustion engines (ICEs), electric drive systems utilizing batteries and/or fuel cells as an energy source, and hybrid systems utilizing a combination of internal combustion engines and electric drive systems. The propulsion systems each have specific technological, financial, and performance advantages and disadvantages, depending on the state of energy prices, energy infrastructure developments, environmental laws, and government incentives.
The increasing demand to improve fuel economy and reduce emissions in present vehicles has led to the development of advanced hybrid vehicles. Hybrid vehicles are classified as vehicles having at least two separate power sources, typically an internal combustion engine and an electric traction motor. Hybrid vehicles, as compared to standard vehicles driven by an ICE, have improved fuel economy and reduced emissions. During varying driving conditions, hybrid vehicles will alternate between separate power sources, depending on the most efficient manner of operation of each power source. For example, a hybrid vehicle equipped with an ICE and an electric motor will shut down the ICE during a stopped or idle condition, allowing the electric motor to propel the vehicle and eventually restart the ICE, improving fuel economy for the hybrid vehicle.
Hybrid vehicles are broadly classified into series or parallel drivetrains, depending upon the configuration of the drivetrains. In a series drivetrain utilizing an ICE and an electric traction motor, only the electric motor drives the wheels of a vehicle. The ICE converts a fuel source to mechanical energy to turn a generator which converts the mechanical energy to electrical energy to drive the electric motor. In a parallel hybrid drivetrain system, two power sources such as an ICE and an electric traction motor operate in parallel to propel a vehicle. Generally, a hybrid vehicle having a parallel drivetrain combines the power and range advantages of a conventional ICE with the efficiency and electrical regeneration capability of an electric motor to increase fuel economy and lower emissions, as compared with a traditional ICE vehicle.
Battery packs having secondary/rechargeable batteries are an important component of hybrid or electrical vehicle systems, as they enable an electric motor/generator (MoGen) to store braking energy in the battery pack during regeneration and charging by the ICE. The MoGen utilizes the stored energy in the battery pack to propel or drive the vehicle when the ICE is not operating. During operation, the ICE will be turned on and off intermittently, according to driving conditions, causing the battery pack to be constantly charged and discharged by the MoGen. The state of charge (SOC, defined as the percentage of the full capacity of a battery that is still available for further discharge) is used to regulate the charging and discharging of the battery.
Rechargeable batteries are also an important component in other applications where a battery pack is continually cycled, such as in solar-powered battery packs for satellites, portable communication apparatus and non-interruptable power supplies.
The preferred embodiment of the present invention utilizes a nickel/metal hydride (NiMH) battery in the battery pack. A NiMH battery stores hydrogen in a metal alloy. When a NiMH cell is charged, hydrogen generated by the cell electrolyte is stored in the metal alloy (M) in the negative electrode. Meanwhile, at the positive electrode, which typically consists of nickel hydroxide loaded in a nickel foam substrate, a hydrogen ion is ejected and the nickel is oxidized to a higher valency. On discharge, the reactions reverse. The reaction at the negative electrode is more clearly shown by the following reaction diagram:
MHx+OHxe2x88x92⇄MHxxe2x88x921+H2O+exe2x88x92
The discharging direction is represented by xe2x86x92; the charging direction is represented by ←.
On discharge, OH- ions are consumed at the negative hydride electrode and generated at the nickel oxide positive electrode. The converse is true for the water molecules.
A difficulty with NiMH batteries is predicting their SOC because of the charging and discharging characteristics of NiMH battery technology.
Referring to FIG. 1, typical charge increasing 10 and charging decreasing 12 curves are illustrated for a NiMH battery. Referencing points A and B and points C and D, it can be shown that the voltages are the same while the SOCs are substantially different. Thus, it is very difficult to use an open circuit voltage to accurately predict the SOC of the NiMH battery, as the battery operating operation (charge increasing, charge sustaining or charge decreasing) must be known. Furthermore, coulombic integration methods to determine the SOC of a battery suffer from accumulated errors. When used with a hybrid vehicle, the intermittent charging and discharging of the battery pack amplifies the problems associated with predicting the SOC of a NiMH battery pack. To successfully operate a hybrid powertrain of a vehicle incorporating a battery pack, an accurate and repeatable estimate of battery SOC is needed.
A technique used by the present invention to describe the SOC and operating characteristics of a battery is state estimation. When a system can be described by either a differential or difference equation, there are state variables associated with that system. These systems are typically referred to as dynamical (or dynamic) systems. A very simple example of such a system comprises:             ⅆ      x              ⅆ      t        =            x      .        =          f      ⁢              (        x        )            
or
xk+1=ƒ(xk)
The notation {dot over (x)} is used to denote differentiation with respect to time. This is a system without an input that has a state x that evolves over time according to the function ƒ. The values at a given time are completely determined by the initial value x0. A more complex system includes inputs and observations (or measurements). An example of such a system is
Xk+1=ƒs(xk,uk)
yk=ƒm(xk,uk)
In this case, u is an input and y is an observable output of the system. ƒs maps the current state, xk, and input, u, to the next state Xk+1. ƒm maps the current state and input to the measurement y. When these equations are used to describe physical systems, noise is added to the description of the system. An example of this is
xk+1=ƒs(xk,uk)+np
yk=ƒm(xk,uk)+nm
In this example, np is the noise associated with the evolution of the state. nm describes noise associated with measurement.
State estimation refers to the set of techniques used to determine the state x given the measurements y for dynamical systems illustrated above. Two very common approaches to state estimation are (a) inversion methods such as {circumflex over (x)}k=ƒmxe2x88x921(y,u) and (b) ad hoc/expert system methods which estimate state based on special case tests or conditions.
For special cases of systems, there are optimal methods for performing state estimation. For linear systems in the presence of white gaussian noise, a Kalman Filter (KF) is the optimal state estimator. The KF estimates the state of the system with the minimum mean squared error between the estimate and the true value. The KF estimates the state using all information available in the model, the noise characterization and all previous observations.
For nonlinear systems, there are several methods available to perform state estimation. The most common of these is the Extended Kalman Filter (EKF) and Hybrid Kalman Filter (HKF). Both linearize the system to update the state estimate. Both are near optimal methods for estimating a systems state.
The present invention includes a method and apparatus to determine the state of charge (SOC) and operating characteristics of a battery pack utilizing NiMH batteries or any other rechargeable battery technology known in the art including, but not limited to, lead acid and lithium polymer batteries. The method of the present invention includes an SOC algorithm that combines the concepts of state estimation with the concepts of system identification in a prediction-correction model. The SOC in a battery and the voltage hysteresis of the battery are estimated as states of the battery. The resistance and impulse response of the battery are estimated using system identification techniques. The state and the system identification are used in combination to establish an optimal estimation of the SOC and other characteristics of a battery.
The SOC algorithm of the present invention relies on a simplified model of the battery behavior that includes an amp-hour integrator, a first order differential equation describing voltage hysteresis, an adaptive description of battery impedance, a function which relates battery charge hysteresis, impedance and current to terminal voltage, and a description of the measurement errors and modeling errors. The model of the battery behavior and description of the measurement and modeling errors are combined into an Extended Kalman Filter (EKF). In operation, the EKF uses the model of the battery to predict the voltage on the terminals due to the current on the terminals. The difference between the predicted voltage and the measured voltage is then used to correct the state variables. The correction to the state variables is a near optimal method for minimizing the sum of squared errors between the estimated values and the actual values.
The present invention, in the preferred embodiment, further includes a vehicle having a parallel hybrid drive system incorporating an energy management controller and hybrid system controller executing the methods of the present invention, an internal combustion engine (ICE), and a motor-generator (MoGen) that charges and discharges the battery pack. The MoGen not only provides for propulsion of the vehicle during certain vehicle operating conditions but also replaces an alternator to charge the battery pack in the vehicle and replaces a conventional starter motor to start the ICE. The hybrid drive system of the present invention will utilize the ICE and MoGen to propel or motor the vehicle during the vehicle conditions which are most efficient for the ICE or MoGen operation. For example, during deceleration or a stopped condition, fuel flow to the ICE will be cut off, as these conditions are some of the least efficient conditions to run the ICE. The MoGen system becomes the active propulsion or motoring system during this fuel cut-off feature and powers the vehicle without noticeably disturbing the operation of the vehicle or sacrificing driveability. The MoGen will propel the vehicle and smoothly transition the vehicle from the idle or stopped state and start the ICE for ICE driving conditions. The transfer of power between the MoGen and ICE or vice versa is transparent to the operator or driver, as the vehicle will perform as if there is only one drive system propelling the vehicle.
During normal operation of the vehicle when the ICE is running, the MoGen will act as an electrical generator to supply electrical power to the vehicle""s electrical infrastructure (fans, radios, instrumentation, control, etc.) as well as recharging the battery pack. The battery pack and a power supply, such as a DCxe2x80x94DC converter, will supply power to the vehicle electrical infrastructure and power the MoGen when it is operating as the motoring device for the vehicle. In the motoring mode, the MoGen is an electrical load drawing current from the battery pack.